asvetlov: Sorry if I articulated myself badly, but I do think this is a valid bug. It's unfortunately hard to provide a better repro (I tried), but we are hitting this regularly when mitmproxy is accepting connections under heavy load. We're just calling `asyncio.start_server(handler, "127.0.0.1", 8080)` in mitmproxy and never interact with the underlying socket object.

Here are some observations that are true for all crashes:

- The socket fileno is -1 when it crashes.
- `_call_connection_lost` is called by `_ProactorBasePipeTransport.close`, which is called by `_ProactorBasePipeTransport.__del__` [1]
- There are no previous calls to `_call_connection_lost`.
- Windows only, loopback connections in our case.
- Wireshark shows that client and server are first happily exchanging packets. At some point the client sends a FIN, which the Python server ACKs immediately. A few seconds later the Python server sends a FIN back.


An obvious fix without understanding the root cause would be to check fileno in https://github.com/python/cpython/blob/d929aa70e2a324ea48fed221c3257f929be05115/Lib/asyncio/proactor_events.py#L161. I'm not too familar with proactor to assess if that is a good idea. Sorry for not being able to provide more details.


[1] https://github.com/python/cpython/blob/d929aa70e2a324ea48fed221c3257f929be05115/Lib/asyncio/proactor_events.py#L102-L116